Wastewater treatment facilities typically utilize naturally-occurring microorganisms to remove dissolved or suspended biological matter from the wastewater to be treated. Typically such facilities include primary, secondary and tertiary treatment systems. Initially, in the primary treatment system, the wastewater is held in a clarifier to allow the heavier solids to settle to the bottom of the basin and lighter materials to rise to the surface. The heavier solids and lighter materials are thereby separated in the primary treatment system and the wastewater removed from such a system can be introduced into the secondary treatment system where the microorganisms can act to remove the remaining biological matter. The effluent from the secondary treatment system is further treated in a tertiary treatment system to make the effluent from the secondary treatment system suitable for discharge into the environment by denitrification and removal of phosphorous, suspended matter, color, etc.
In a secondary treatment system utilizing activated sludge, the effluent from the primary treatment system is introduced into an aeration basin along with oxygen to support bacterial activity. The effluent from the aeration basin is then introduced into a clarifier tank or a series of such tanks or a membrane bioreactor. A return sludge stream is recycled from the clarifier or membrane bioreactor back to the aeration basin to provide some of the bacterial activity within the aeration basin. In a clarifier, the sludge settles to the bottom of the tank and the return sludge stream is circulated from such a tank or basin back to the aeration basin. In a membrane bioreactor the mixed liquor is forced through a porous membrane that has pores to allow the effluent to be filtered to produce a permeate which can be discharged, or sent for further processing and a retentate from which the return activated sludge stream is recirculated back to the aeration basin. Further aeration and oxygen addition may be present within such a reactor to prevent fouling of the membrane.
It is to be noted that membrane bioreactor use is growing in water and wastewater treatment because such systems provide higher quality effluent and can be operated at higher mixed liquor suspended solids concentrations than conventional, gravity based secondary clarification processes. The ability to operate at higher mixed liquor suspended solids concentration allows for a smaller footprint of the wastewater treatment system and can result in lower capital costs and energy requirements. In conventional wastewater treatment processes using gravity settling clarifiers, the level of mixed liquor suspended solids is limited by the surface overflow rate of the clarifier and the solids loading rate to the clarifier. A membrane bioreactor has no such limitation to its solids loading capacity and therefore, such bioreactors can be operated at higher solids loadings than conventional secondary clarifiers. However, fouling is a major issue with membrane bioreactor systems because such fouling will reduce membrane flux and permeability, and increase the energy consumed by the membrane. Fouling also leads to an increase in the frequency of cleaning required to obtain an acceptable flux. Fouling can be a result of extracellular polymeric substances that are routinely released by biological cells in an activated sludge process. Such substances consist mainly of a mixture of proteins, polysaccharides, lipids, nucleic acids and humic compounds. Microorganism stress will increase the release of such substances. Additionally, such stress will also lead to the formation of filamentous microorganism growth or bulking which is also a source of fouling. It is to be mentioned, however, that the production of extracellular substances and bulking is also a problem in conventional secondary clarifiers because such formation will have an effect on the clarifier effluent and therefore, overall system performance.
Low dissolved oxygen levels within the mixed liquor of the aeration basin are a source of microorganism stress that can lead to the production of extracellular substances and bulking In accordance with the present invention, the oxygen addition into the aeration basin is controlled to prevent such microorganism stress. However, the control of dissolved oxygen levels within the aeration basin is not completely straightforward because the flow of effluent passing through the secondary treatment system can vary as a result of many known factors, for instance fluctuations in water usage, varying organic loads in the influent flow and increased flows produced by run-off. Moreover, instrumentation that is used will often not accurately reflect the level of dissolved oxygen within the mixed liquor contained in the aeration basin.
As will be discussed, the present invention provides a method and system for introducing oxygen into an aeration basin of a secondary treatment system which among other advantages provides a high degree of assurance that the dissolved oxygen will not fall to a level that is sufficiently low to cause stress in the bacterial environment and resulting in bulking and formation of extra cellular polymeric substances.